Scatterometers are used in many industrial applications from pharmaceuticals, paint, food production, and surface inspection owing to the ability to quickly and reliably characterize the size and distribution of microscopic particles. Scatterometers obviate the need for direct imaging to characterize optical or geometric properties of single or multiple scatterers. Typical devices measure the intensity of light as a function of position (e.g., angle) at a given distance from the scattering sample. This measurement, called the scattering spectrum, is typically made by use of a collimated or quasi-collimated laser beam or other quasi-monochromatic light source. Typical devices are unable to accurately determine scattering spectra at very low angles where the unscattered light is much brighter than the scattered light.
Intensity measurements of scattered laser light at various angles (the angular spectrum) may be used to measure surface contamination, characterize the clarity of transparent solids, evaluate the bidirectional reflectance distribution function (BRDF), or determine the sizes of particles suspended in a liquid or solid. Size determination may be made for particles of a known refractive index by fitting the measured angular spectrum data to models based on Mie scattering theory, and thus, it is important to record a full distribution of angles. However, in the weak scattering regime typical of this method, it is difficult to distinguish low-angle scattered light from the more intense unscattered laser light, especially near the zero angle. That is, the signal is lost in the glare of the illumination source. This resulted in inaccurate low-angle scattering values or estimates which are used to approximate the needed data.
The art lacks the ability to accurately measure the full scattering spectrum, particularly at very low or zero scattering angles.